The present invention relates to a suspension arm for an automobile manufactured by forging.
Conventionally, a suspension arm for an automobile having an arm section of an L-shape in a plan view is used for a strut type suspension and a double wishbone type suspension (automobile chassis component), and is known to have the advantage of improving the riding quality and the traveling stability and reducing the upsprung weight (refer to JP-A H5-112111, JP-A 2004-299663, and JP-A 2005-082140).
With respect to the suspension arm for an automobile having an arm section of an L-shape described above, it is known that an H-shape cross-sectional structure that can increase the in-plane bending rigidity is adopted so as to sufficiently stand the in-plane moment inevitably applied to the arm section of an L-shape (which is the moment applied within a plane perpendicular to the vertical direction of the vehicle) because the input load in the longitudinal direction of the automobile (will be hereinafter also referred to as “vehicle”) is most severe.
Also, as a suspension arm for an automobile aiming to achieve substantial weight reduction while securing the strength, one employing an arm section of an L-shape using nodular graphite cast iron material and having a channel shape cross-sectional structure is also known (refer to JP-A 2002-307921).
However, the suspension arm for an automobile employing the arm section of an L-shape having the H-shape cross-sectional structure described above had a problem that although it was designed considering the vibration mode by the in-plane moment generated at a natural frequency of a comparatively high order (that is, having sufficient in-plane bending rigidity), it was not designed sufficiently considering the vibration mode by the out-of-plane moment generated mainly at a natural frequency of a low order (first order and second order for example). That is, there was a problem that the out-of-plane bending rigidity (strength) was insufficient.
Also, the suspension arm for an automobile employing the arm section of an L-shape having a channel shape cross-sectional structure disclosed in JP-A 2002-307921 described above employed a cross-sectional structure that couldn't be manufactured by forging cost effectively, and therefore there was a problem that it was imperative to manufacture the same by casting.